Conventional diamond FETs are fabricated using a p-type semiconductor layer made by doping boron atoms (B) into a diamond layer (K. Kobashi, K. Nishimura, K. Miyata, R. Nakamura, H. Koyama, K. Saito and D.L. Dreifus: Proc. 2.sup.nd Int. Conf., Appl. Of Diamond Films and Related Materials., ed. M. Yoshikawa, M. Murakawa, Y. Tzeng and W. A. Yarbrough, pp 35-42 (Saitama, 1993)).
FIG. 10 is a schematic plan view of a conventional FET mentioned above, and FIG. 11 is a fragmentary cross-sectional view taken along the B--B line of FIG. 10. FET 80 is made by first depositing a diamond layer 82 on a substrate 81 with no impurity doped and a boron-doped layer 83 on the diamond layer 82, then making an annular p.sup.+ region 84, a circular p.sup.+ region 85 concentric with the annular region 84 and an annular non-doped region 86 between the annular region 84 and the circular region 85, further making a source ohmic contact 87, a drain ohmic contact 88 or a gate electrode 89 on the respective regions. That is, this FET requires a circular geometry of elements as shown in FIG. 10 to prevent leakage of a current to the exterior, and does not suit for microminiaturization or for integration of such elements with a high density.
In regard of Si semiconductors, LOCOS technique is used as an element insulating technique for insulating and isolating elements from each other by providing an oxide layer as shown in, for example, Japanese Patent Publication S49-45629. The LOCOS technique, however, cannot be used for diamond semiconductor devices because diamond itself is not readily oxidized thick enough to form an oxide film on the surface of the diamond. An alternative approach for isolating elements on a diamond semiconductor device selectively removes semiconductor regions by etching, etc. to isolate respective elements on a diamond substrate as taught in, for example, "Diamond Thin-Film Recessed Gate Field-Effect Transistors Fabricated by Electron Cyclotron Resonance Plasma Etching: S. A. Grot, G. S. Gildenblat, and A. R. Badzian, IEEE ELECTRON DEVICE LETT. VOL 13 No.9, September 1992, pp 462-463". This approach, however, requires complicated processes including etching.
The present inventors formerly proposed at Applied Physics Conference a method for fabricating MESFET including a source ohmic contact and a drain ohmic contact made by vapor deposition of aluminum (Aoki, Ito, Kawarada, et al.: Brief of 40.sup.th Joint Meeting Related to Applied Physics, p. 30, M11, 12). This method certainly provides MESFETs having a simple structure and operative in an enhanced mode; however, since a surface conductive layer is formed directly under the hydrogen-terminated surface of the homoepitaxial diamond, even this method failed to insulate respective elements, and could not integrate a plurality of semiconductor elements on a common substrate.
In order to solve this problem, the present applicant filed a patent application regarding the invention to isolate elements in a hydrogen-terminated diamond semiconductor device (Japanese Patent Application No. H7-64035, Japanese Patent Laid-Open No. H8-139109). This invention relates to a hydrogen-terminated diamond semiconductor device having isolated elements comprising a hydrogen-terminated region and a non-hydrogen-terminated region on the diamond surface, mounting a semiconductor element on the hydrogen-terminated region, and further relates to an enhancement-type MESFET.
However, when forming an inverter circuit using an enhancement-type MESFET as a load, it will not work as a load when the gate and the source is connected, because the current will not flow at Vgs=0 V. Therefore, when the circuit having an EE (enhancement enhancement) structure wherein two enhancement-type MESFETs are connected in series, the drain and the gate is connected regarding the load, but the output voltage will reduce by the threshold voltage, lowering the utilization ratio of the power source. Therefore, it was necessary to operate it with higher power source voltage.
The present invention is aimed at providing a MISFET formed on the surface of a hydrogen-terminated diamond.
Further, the present invention is aimed at providing a simple method of manufacturing a semiconductor device wherein each element formed on the surface of the same hydrogen-terminated homoepitaxial diamond substrate with a reduced number of mask processing.
The further object of the present invention is to provide a hydrogen-terminated diamond MISFET providing an FET having any mode of the depletion mode or the enhancement mode.